For example, International Publication No. WO2011/043354 discloses a displacement measurement apparatus that uses an optical interference. The displacement measurement apparatus includes, in the stated order from a light source side, a laser light source, a collimator lens, a first diffraction grating, a second diffraction grating, and an optical sensor. The optical sensor detects diffracted light (e.g., 1-order) diffracted by the first diffraction grating and interference light with respect to diffracted light (e.g., 1-order) that is caused as 0-order light that has passed directly through the first diffraction grating is diffracted by the second diffraction grating. The displacement measurement apparatus measures a distance between the first diffraction grating and the second diffraction grating, that is, a displacement of a measurement target, based on a change in a light amount due to brightness/darkness of the interference light detected by the optical sensor (see, for example, paragraphs [0020], [0023], and [0027] and FIG. 1 to 3 of International Publication No. WO2011/043354).